Evolution of an Intra-Slope Apron, Offshore Niger Delta Slope: Impact of Step Geometry on Apron Architecture
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Mark D. Barton, 2012. "Evolution of an Intra-Slope Apron, Offshore Niger Delta Slope: Impact of Step Geometry on Apron Architecture", Application of the Principles of Seismic Geomorphology to Continental Slope and Base-of-Slope Systems: Case Studies from SeaFloor and Near-Sea Floor Analogues, Bradford E. Prather, Mark E. Deptuck, David Mohrig, Berend Van Hoorn, Russell B. Wynn
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A high-resolution 3-D seismic dataset from the offshore Niger delta slope was utilized to study the stratigraphic architecture and evolution of a near-seafloor intraslope apron that overlies an abrupt break in slope. Elements that constitute the apron are from oldest to youngest: (1) a package of prograding lobes, (2) a complex of laterally offset stacked channels, and (3) a sinuous deeply incised bypass channel. Apron evolution reflects the adjustment and response of sediment gravity flows to an evolving slope gradient. Lobes are deposited as flows enter the basin and encounter an abrupt decrease in slope, decelerate, and lose confinement. As the step is healed, flows remain confined and form channels. Eventually, the apron becomes a site of erosion and bypass as down-dip basins become linked by a common graded profile. A comparison with published examples of slope aprons suggests that the geometry of the step may impact the architecture of the apron. Aprons formed above mild breaks in slopes should be thinner, more channelized, and potentially more dissected then aprons formed above severe breaks in slope.
Intraslope basins are sites where sand-prone deposits accumulate; they represent a class of economically important hydrocarbon plays around the world (Prather, 2003). Recent sea-floor and near-sea-floor studies of submarine slope fans and aprons have improved our understanding of their gross morphology, evolution, and controls on deposition (Beaubouef and Friedmann, 2000; Fonnesu, 2003; Prather and Pirmez, 2003; Adeogba et al. 2003). However, the internal architecture and depositional elements that comprise stepped slope aprons is not well documented. The objective of this study is to use a high-resolution three-dimensional (3-D) seismic data set to characterize the internal architecture of a near-sea-floor apron that occupies a shallow, stepped basin located along the western Niger delta slope. Deepwater outcrop analogues from similar depositional settings are integrated with the seismic-based interpretations to provide a picture of the geometry, connectivity, and facies architecture of these deposits that is below the resolution of the seismic. This information can be used to better characterize the internal architecture of less well-imaged reservoirs that may have formed in comparable depositional settings.
The Niger Delta, located along the western margin of Africa, forms a symmetrical protrusion into the Gulf of Guinea that covers area of about 210,000 km2 and reaches a maximum thickness of about 12 km (Damuth, 1994). It consists of a regressive sequence of Tertiary clastics that prograded over a passive-continental-margin sequence of mainly Cretaceous sediments. (Doust and Omatsaola, 1990). The submarine slope starts 50–80 km offshore of the coastline at a water depth of about 200 m. Sediment delivered to the submarine slope are sourced from the delta as well as embayments that flank the margins of the delta (Burke, 1972).
Due to rapid sedimentation, the Niger Delta is the site of active loading, growth faulting, and shale remobilization at depth (Allen, 1965; Damuth, 1994; Pirmez et al., 2000; Steffens et al., 2003). As a result, the slope consists of alternating bathymetric highs and lows that (1) affect the path of sediment gravity flows passing through the slope, and (2) act as sediment traps (Prather 2000, 2003). Prather (2003) further classified the Niger Delta slope as an “above-grade, slope that exhibits subtle changes in depositional gradient resulting in low-relief stepped or terraced topography”. The study focused on a shallow intraslope basin located 250 km northwest of the present Niger Delta (Fig. 1). The area covers a 25 km by 15 km region (375 km2) positioned about 80 km downslope from the shelf break in a mid- to lower-slope setting. Water depth increases from approximately 2200 m in the east to over 2800 m in the west.
Data and Methods
The interval studied was within the upper 350 ms of strata and extended from the sea floor down to a base horizon that extended across the study area. Additional horizons subdivide the interval into major packages. Due to the complexity of the events, each horizon was mapped along every in-line (25 m spacing) for the length of the survey. Amplitude and isochore maps were extracted for each horizon and the intervals between them. Though there are no well data to calibrate the seismic response, high amplitudes in the image (red/yellow) are interpreted to result from high-impedance material, i.e., sands, and zero-crossings in the runsum volumes are interpreted to correspond roughly to contacts between sandstone and mudstone units. The seismic volume has a frequency near 65 Hz and an inline and cross-line spacing of 25 m ×37.5 m. Estimated vertical resolution is about 8 m. Outcrop analogues presented include following: (1) Colleen Canyon, Brushy Canyon Formation, west Texas; (2) Willow Mountain, Bell Canyon Formation, West Texas; (3) Popo Channel, Brushy Canyon Formation, west Texas, (4) Plane Crash Canyon, Brushy Canyon Formation, west Texas, and (5) the Condor West Channel, Cerro Toro Formation, southern Chile.
The base horizon is the reflector that defines the base of the system studied. It represents a relatively continuous event on which subsequent events onlap or downlap. The horizon displays a complex physiography composed of high-relief, discontinuous, mud-cored ridges that trend parallel to the dip in slope and low-relief steps that develop orthogonal to the ridges (Fig. 2). Amplitude maps above and below the horizon indicate that the ridges form corridors that funnel sediment into distinct channel fairways that shift location through time (Fig. 3). Below the base horizon, corridor three is occupied by a series of sinuous channel systems while corridors one, two, and four appear inactive. Above the base horizon, corridors one, two, and four are occupied by sinuous channel systems whereas corridor three appears to be inactive. In corridor four a fan-like apron deposit is present beneath the channel systems. This deposit was the primary focus of this study (Fig. 4). An abrupt break in slope separates corridor four into a pair of steps (referred to as steps one and two), defined as the relatively-flat lying portions of the slope, separated by a shallow ramp, defined as the relatively steep portion of the slope (Fig. 5). The lowermost step (step two) covers an area that is about 18 km in length (in a down-dip direction) and 12 km in width (in a direction parallel to the regional slope). It displays an average gradient of around 1.0°. The corresponding ramp displays an average gradient of about 4.0° and a lateral extent of 3–4 km. Steps one and two are linked by a narrow incised channel that is around 400 m in width and up to 30 ms in depth. The fill above the base horizon consists of two main elements: (1) an older, relatively thin (less the 90 ms), funnel-shaped sediment wedge, referred to as a slope apron, and (2) a younger, thicker interval (up to 350 ms) composed of a divergent network of channel–levee complexes.
Apron Architecture and Evolution
The slope apron originates from the incised-channel system located along the ramp between steps one and two narrow (Fig. 6). It is about 10 km in width and 16 km in length. Maximum thickness of the apron is about 90 ms and occurs near the entry point at the transition from ramp to step. The apron spreads out and thins down dip. Internally, it is composed of thicks and thins that display a divergent pattern. In addition, portions of the apron, including the primary exit point for flows leaving step 2, have been erosionally replaced by younger channel–levee systems. The northern portion of the apron is significantly thicker (by about 20 ms) then the southern portion. In cross section the apron displays a wedge geometry that overall thins down dip (Fig. 7). Large portions of the apron have been completely removed by erosion from overlying channel–levee systems. Reflector amplitude and continuity are variable but tend to increase in a down-dip direction within the apron. In strike view, reflectors in the apron converge to the south and on lap to the north (Fig. 8). Their amplitude and continuity appear greater in the southern half of the apron, whereas the base horizon appears more irregular and erosional in the northern half of the apron. Minimum-amplitude maps for the base and top apron horizon are shown in Figure 9. The dimmest amplitudes occur near the sediment entry point and down the axis of the system, whereas the brightest amplitudes are distributed along the flanks of the system.
Based on reflector patterns, amplitudes, and bounding surfaces the apron is subdivided into three distinct packages. Each package represents a discrete phase of deposition, which from oldest to youngest are referred to as (1) a lobe-dominated package, (2) a channel-dominated package, and (3) a bypass-channel package (Fig. 10). The lobe-dominated package dominates the southern and eastern portions of the apron and consists of moderate- to high-amplitude reflectors that display moderate to high continuity. Its basal surface appears conformable to slightly erosional. The channel-dominated package is restricted to the northern part of the apron. The basal surface appears erosional. Reflector amplitude is more variable and less continuous than in the lobe-dominated package. The bypass-channel system is incised into the channel-dominated package to the north and the lobe-dominated package to the south. Reflectors are relatively dim in comparison to the other packages. By volume, the lobe-dominated package makes up about 60 percent of the apron, the channel-dominated package 25 percent, and the bypass channel 15 percent. In a strike sense the packages are laterally offset, with little overlap existing between adjacent packages.
The lobe-dominated package shows a progressive thinning and spreading from east to west. Reflectors display divergent ribbonlike patterns, interpreted as distributary channels, that pass down dip into broad, fan-shaped amplitudes, interpreted as lobes (Fig. 11). There are ten individual lobe elements. Individual lobe elements are 1–4 km in width and 2–6 km in length. The elements prograde or step basinward, with younger lobe elements often appearing to incise into older lobe elements, especially near the proximal portion of the apron. Each lobe element is associated with a distributive channel system that branches off from a larger, long-reach channel interpreted as a feeder channel (Fig. 11). In cross section the channels display a cuplike geometry near the up-dip portion of the apron and a lens-shaped geometry near the down-dip portion (Fig. 11). The short-reach, radially arranged channels are around 100–200 m in width. The larger, long-reach channels are 400–500 m in width. Channels often display dimmer amplitudes then associated lobe elements, suggesting that they may not be as sand-rich. The larger, long-reach channels are occasionally flanked by broad, apron-shaped elements interpreted as a lateral (Figs. 12, 13). Individual splays range from 1 to 2 km in width and from 2 to 3 km in length.
The channel-dominated package displays moderate- to high-amplitude reflectors with low continuity (Fig. 14). The package is restricted to the northern half of the apron and displays a width of 2 to 3 km and a thickness that varies from 60 to 80 ms. Within the package, reflector terminations map out as ribbonlike elements that overall trend from west to east (Fig. 15). Several of the elements display arcuate geometries. The elements are interpreted as crosscutting channel elements that form a complex of highly amalgamated channels. The reflector terminations result from the onlap of channel fills onto channel margins, and the truncation of older channel by a younger channel. Individual channel elements range in width from 500 to 750 m. The brightest amplitudes occur near the base of the channels and may represent coarse-grained lag deposits. Dim amplitudes occur near the edges of the channels and may represent fine-grained channel-margin or overbank deposits.
The bypass channel is a deeply incised, throughgoing channel system that dissects the apron into a northern channel-dominated package and a southern lobe-dominated package (Figs. 16, 17). The package is up to 2 km in width and 90 ms in depth. It consists of three parts: (1) a narrow, sinuous, low-amplitude, throughgoing channel element, (2) a series of linear- to pod-shaped high-amplitude reflectors located at the inside bends of the sinuous channel, and (3) a region of chaotic, low-amplitude reflectors that flank the margins of the sinuous channel. The sinuous channel element is 300 to 400 m in width. Depth increases in a down-dip direction from about 40 ms at the entry point to about 80 ms at the exit point. About two kilometers down dip from the entry point the depth of the channel abruptly increases from about 45 to 65 ms. The abrupt increase in incision is interpreted as an up-dip-migrating knickpoint. The knickpoint represents a drop in base level and readjustment of the equilibrium profile through erosion and the base of the channel (Adeogba, 2003). Up dip of the knickpoint, the channel is relatively straight, while down dip of the knickpoint it becomes progressively more sinuous. The pattern suggests that the up-dip portion of the channel was deepening by way of knickpoint migration and erosion, while the down-dip portion of the channel had achieved base level and was widening by lateral channel migration. The high-amplitude pods, present at the base of the bypass-channel package, are deposits of the laterally migrating channel. In cross section, high-angle truncations can be mapped that follow the direction of channel migration (Fig. 17B). The deposits are believed to consist of very coarse-grained material, reworked from previous sediments, by flows that for the most part passed though the system. Chaotic, low-amplitude reflectors that flank the channel are interpreted as fine-grained slump and inner-levee deposits. The depth of incision and the presence of a throughgoing channel is evidence of sediment bypass to the next sub-basin lower on the slope (Adeogba, 2003).
Channel–Levee Architecture and Evolution
The interval between the sea floor and the top apron horizon consists of a distributive network of channel–levee complexes (Figs. 18, 19). The channel–levee complexes are up to 300 ms in thickness (measured from base of channel to levee crest) and 2– 4 km in width (measured between levee crests). Individual channel elements are 200–500 m in width and highly sinuous. Cross-cutting relationships indicate that the channel–levee complexes are not contemporaneous but rather separate events related to abrupt lateral shifts in the position of the channel through time due to avulsion. Three main channel–levee complexes, numbered 1 through 3 in ascending stratigraphic order, are identified. The position of the channels appears to be controlled by topography, with channels converging across the ramp and diverging across the step. Avulsion points are located near breaks in slope. The first avulsion point occurs near the western edge of step 1 and results in channel system (CLC-1) being diverted to the north of the apron bypass channel. The second and third avulsion points occur at the ramp-to-step transition above step 2 and result in channel systems CLC-2 and CLC-3 being diverted to the south. The avulsions also appear to occur after a period of channel aggradation. Within channel–levee elements 1 and 2, the elevation of the final active channel fill is about 50 ms below the levee crest and nearly 250 ms above the erosional base of the system (Fig. 15B). Each of the channel systems (CLC-1, CLC-2, and CLC-3) incise through the underlying apron. HARP-like deposits, originally defined as high-amplitude reflection packets (Pirmez et. al., 1997), are associated with channel–levee complex 3. The deposits display funnel-shaped geometry, up to 100 ms in thickness and 8 km in width. They are confined by an anticlinal structure to the south and by topography created by channel– levee complex 2 to the north. The internal architecture of the deposits was not investigated, but the base looks erosional and the internal character appears channelized.
The slope apron at OPL315 evolved in three distinct phases that correspond to a lobe-dominated package, a channel-dominated package, and bypass-channel system. Each phase is interpreted to reflect the adjustment and response of sediment gravity flows to an evolving slope gradient (Fig. 20). Deposition began after subsidence and the formation of a shallow, stepped basin. A lobe-dominated slope apron is formed as originally confined sediment gravity flows encounter an abrupt decrease in slope, decelerate, lose confinement, and deposit their load. As the slope break is healed, and a local graded profile achieved, apron aggradation ceases. Incoming sediment gravity flows remain confined, eroding and replacing portions of the apron by channels. Over time, down-dip basins become linked by a regional or common graded profile and the step becomes a site of erosion and bypass. Flows consolidate into a single throughgoing bypass channel that shows evidence of significant incision and enlargement. The model is similar to previously described models for slope aprons (Beaubouef and Friedmann, 2000; Prather, 2003; Adeogba, 2003). The primary difference is the development of a local graded profile that results in significant portions of the apron being channelized.
A comparison with published examples of slope aprons, such as OPL 211 (Prather et al., 2007) shows significant differences in geometry and architecture (Fig. 21). Differences include: (1) the geometry of the ramp and step, (2) the stacking pattern of depositional units, (3) internal architectures, and (4) the degree of dissection. At OPL 211 the ramp is steeper (80 ms/km vs. 55 ms/km), the step flatter (0 ms/km vs. 10 ms/km), and the apron thicker (120 ms vs. 80 ms), than at OPL 315. At OPL 211 the apron is made up of vertically stacked, progradational units. At OPL 315, the units erode and stack in a lateral fashion. Internally, depositional units at OPL 211 consist of distributive channel–lobe complexes. At OPL 315, initial depositional units consist of distributive channel–lobe complexes while later units consist of channel complexes. The apron at OPL 315 is completely dissected by the bypass channel, whereas dissection of the OPL 211 apron is focused near the entry and exit points.
Differences in element geometries and stacking patterns between the two systems may result from variations in the steepness of the slope and corresponding step (Fig. 22). Sediment gravity flows encountering a sharp break in slope, such as OPL 211, which displays a steep slope passing into a flat step, are likely to transfer a larger component of the flow energy laterally than sediment gravity flows encountering a mild or shallow break in slope, such as OPL 315, which displays a shallow slope passing into a slightly dipping step. In the OPL 211 case, the greater lateral transfer of energy at the slope break causes the flow to lose confinement, channels become unstable, and lobes are deposited in an aggradational to laterally stacked fashion across the break in slope. By contrast, in the OPL 315 case, the relatively shallow break in slope allows flow energy to be transferred basinward. As a result, the flow remains confined, channels maintain stability, and lobes are deposited in a progradational fashion across the step.
Two types of systems are seen across the step slope in the study area: a slope apron, and a distributive network of channel–levee complexes. Deposition of the slope apron began after subsidence and the formation of a shallow stepped basin. The apron consists of three distinct phases of sedimentation: a lobedominated package, a channel-dominated package, and a bypass channel. A lobe-dominated slope apron is formed as originally confined sediment gravity flows encounter an abrupt decrease in slope, decelerate, lose confinement, and deposit their load. As the slope break is healed, and a local graded profile achieved, apron aggradation ceases. Incoming sediment gravity flows remain confined, eroding and replacing portions of the apron by channels. Over time, down-dip basins become linked by a regional or common graded profile and the step becomes a site of erosion and bypass. Flows consolidate into a single throughgoing bypass channel that shows evidence of significant incision and enlargement. Differences in apron architecture may reflect difference in the geometry of the step. Aprons formed above mild breaks in slopes should be thinner, more channelized, and potentially more dissected than aprons formed above severe breaks in slope.
I thank Petroleum Geo-Services for allowing seismic data utilized in this study to be published. The paper benefited greatly from discussions with Alesandro Cantelli, Carlos Pirmez, Brad Prather, and Ciaran O’Byrne. Support for this project provided by Shell Exploration and Production, including managers Steve Tennant, Mark Hempton, Steve Naruk, and Marc Alberts.
Figures & Tables
Application of the Principles of Seismic Geomorphology to Continental Slope and Base-of-Slope Systems: Case Studies from SeaFloor and Near-Sea Floor Analogues
The study of near-seafloor deepwater landscapes and the processes that form them are as important to the understanding of deeply buried marine depositional systems as the study of modern fluvial environments is to our understanding of ancient terrestrial depositional systems. In fact, these near-seafloor studies follow in the great tradition established by earlier clastic sedimentologists in the use of modern systems to understand ancient environments. The acquisition and mapping of exploration 3D seismic surveys over the last few decades allows for the study of seafloor geomorphology with a spatial resolution comparable to most deepwater multibeam bathymetric tools, and represents a significant advancement that can be used to push forward general understanding of slope and base-of-slope depositional systems through the application of the emerging science of seismic geomorphology. The papers assembled for this volume demonstrate the utility of seafloor-to-shallow subsurface data sets in studying the development of submarine landscapes and their affiliated sedimentary deposits. These contributions highlight the controls of slope morphology on patterns of both sedimentation and erosion. Many of the papers also highlight the influence of pre-existing seafloor relief on confining sediment-gravity flows specific transport pathways, thereby affecting subsequent evolution of the seafloor. The understanding of depositional processes that comes from studying deepwater analogue systems remains the best way take to knowledge from one basin or system and apply confidently to another for prediction and characterization of reservoirs for exploration and production of hydrocarbons.